Auto merge of #95880 - cjgillot:def-ident-span, r=petrochenkov

Handle `def_ident_span` like `def_span`.

`def_ident_span` had an ad-hoc status in the compiler.

This PR refactors it to be a first-class citizen like `def_span`:
- it gets encoded in the main metadata loop, instead of the visitor;
- its implementation is updated to mirror the one of `def_span`.

We do not remove the `Option` in the return type, since some items do not have an ident, AnonConsts for instance.

Author: bors